The present invention relates to a gas flow measuring apparatus, or more in particular to an apparatus for measuring the flow rate of the engine intake air.
Conventional apparatus have been known, which include a flow rate measuring tube in the intake pipe, an electric heater in the flow rate measuring tube, and a pair of temperature-dependent resistors disposed before and after the electric heater, thereby to measure the flow rate of the intake air in response to the outputs of the resistors.
These conventional apparatus have the advantage that the measuring accuracy is not changed by the mounting position due to a compact and simple construction thereof. Nevertheless, a DC voltage is applied to a bridge including the two temperature-dependent resistors and two reference resistors, and a voltage between a pair of diagonal points of the bridge is amplified by a DC differential amplifier, and a voltage or current applied to the electric heater is controlled in accordance with the amplified voltage for measuring the flow rate. As a result, a temperature drift occurs in the DC differential amplifier, thus deteriorating the measuring accuracy of the apparatus. This inconvenience may be improved to some degree by using an element of low drift, but the improvement is not satisfactory on the one hand and the resulting apparatus is higher in cost than the one using a multi-purpose element on the other hand.
Another apparatus is proposed, for example, in U.S. Pat. No. 4,275,590 issued on June 30, 1981 and assigned to the same assignee as the instant application, which is intended to obviate this shortcoming and includes an oscillator circuit for producing a pulse signal, an AC differential amplifier circuit for differentially amplifying a voltage between a pair of diagonal points of the bridge, a rectifier circuit for rectifying the output voltage from the AC differential amplifier circuit, a sample-hold circuit for sampling and holding a sample voltage for only a part of a period of the output voltage of the rectifier circuit synchronous with the pulse signal of the oscillator circuit, a DC differential amplifier circuit for differentially amplifying the output voltage of the sample-hold circuit and a predetermined reference voltage, a power amplifier circuit for amplifying the output voltage of the DC differential amplifier circuit and controlling the voltage applied to the electric heater, and a chopper circuit for interrupting the output voltage of the power amplifier circuit in synchronism with the pulse signal of the oscillator circuit and applying the resulting voltage to the bridge. In this apparatus, the AC amplifier for amplifying the output signal of the bridge has a differential signal input coupled by a capacitor. As a result, in order to secure a large common-mode rejection ratio (CMRR) of the AC amplifier, a feedback resistor, a grounding resistor and an input resistor connected to the differential amplifier used with the AC amplifier are required to have a highly accurate resistance value, thereby leading to a high cost. Further, if the long-term stability can not be assured in the relative value of the leakage current or the capacitance of the coupling capacitor for the differential input, the measuring accuracy of the apparatus will be reduced by the secular variation. For this reason, a high accuracy of the coupling capacitor is required.